Cells infected with herpes simplex virus 1 export to uninfected cells exosomes containing STING, viral mRNAs, and microRNAs

Extracellular Vesicles in Viral Pathogenesis: A Case of Dr. Jekyll and Mr. Hyde

Secretion of extracellular vesicles (EVs) is a fundamental property of living cells. EVs are known to transfer biological signals between cells and thus regulate the functional state of recipient cells. Such vesicles mediate the intercellular transport of many biologically active molecules (proteins, nucleic acids, specific lipids) and participate in regulation of key physiological processes. In addition, EVs are involved in the pathogenesis of multiple diseases: infectious, neurodegenerative, and oncological. The current EV classification into microvesicles, apoptotic bodies, and exosomes is based on their size, pathways of cellular biogenesis, and molecular composition. This review is focused on analysis of the role of EVs (mainly exosomes) in the pathogenesis of viral infection. We briefly characterize the biogenesis and molecular composition of various EV types. Then, we consider EV-mediated pro- and anti-viral mechanisms. EV secretion by infected cells can be an important factor of virus spread in target cell populations, or a protective factor limiting viral invasion. The data discussed in this review, on the effect of EV secretion by infected cells on processes in neighboring cells and on immune cells, are of high significance in the search for new therapeutic approaches and for design of new generations of vaccines.

Lymphocyte Changes in Severe COVID-19: Delayed Over-Activation of STING?

Upon recognition of microbial DNA or self-DNA, the cyclic-GMP-AMP synthase (cGAS) of the host catalyzes the production of the cyclic dinucleotide cGAMP. cGAMP is the main activator of STING, stimulator of interferon genes, leading to interferon synthesis through the STING-TBK1-IRF3 pathway. STING is also a hub for activation of NF-κB and autophagy. The present review details the striking similarities between T and B cell responses in severe coronavirus disease 2019 (COVID-19) and both animal or human models of STING gain of function (SAVI syndromes: STING-associated vasculopathy with onset in infancy). Those similarities may be further clues for a delayed activation of STING in severe COVID-19 patients, due to DNA damages following severe acute respiratory syndrome coronaviruses (SARS-CoV-2) infection and unusual role of STING in SARS-CoV-2 control. In early stages, Th2 differentiation are noticed in both severe COVID-19 and SAVI syndromes; then, CD4+ and CD8+ T cells functional exhaustion/senescent patterns due to TCR hyper-responsiveness are observed. T cell delayed over-responses can contribute to pneumonitis and delayed cytokine secretion with over-production of IL-6. Last, STING over-activation induces progressive CD4+ and CD8+ T lymphopenia in SAVI syndromes, which parallels what is observed in severe COVID-19. ACE2, the main receptor of SARS-CoV-2, is rarely expressed in immune cells, and it has not been yet proven that some human lymphocytes could be infected by SARS-CoV-2 through CD147 or CD26. However, STING, expressed in humans T cells, might be triggered following excessive transfer of cGAMP from infected antigen presenting cells into activated CD4+ and CD8+ T cells lymphocytes. Indeed, those lymphocytes highly express the cGAMP importer SLC19A1. Whereas STING is not expressed in human B cells, B cells counts are much less affected, either in COVID-19 or SAVI syndromes. The recognition of delayed STING over-activation in severe COVID-19 patients could prompt to target STING with specific small molecules inhibitors already designed and/or aspirin, which inhibits cGAS.

The Role of Extracellular Vesicles in Demyelination of the Central Nervous System

It is being increasingly demonstrated that extracellular vesicles (EVs) are deeply involved in the physiology of the central nervous system (CNS). Processes such as synaptic activity, neuron-glia communication, myelination and immune response are modulated by EVs. Likewise, these vesicles may participate in many pathological processes, both as triggers of disease or, on the contrary, as mechanisms of repair. EVs play relevant roles in neurodegenerative disorders such as Alzheimer’s or Parkinson’s diseases, in viral infections of the CNS and in demyelinating pathologies such as multiple sclerosis (MS). This review describes the involvement of these membrane vesicles in major demyelinating diseases, including MS, neuromyelitis optica, progressive multifocal leukoencephalopathy and demyelination associated to herpesviruses.

Integrity of the Antiviral STING-mediated DNA Sensing in Tumor Cells Is Required to Sustain the Immunotherapeutic Efficacy of Herpes Simplex Oncolytic Virus

Simple Summary

Oncolytic viruses are emerging immunotherapeutics in cancer treatments. The conflicting role of innate immunity in the antitumor activity of oncolytic viruses is still a matter of debate. The STING-dependent DNA sensing axis is considered detrimental for viral replication and cancer cell clearance. Accordingly, we observed that STING loss in tumor cells was associated with improved lytic potential by a herpes-based oncolytic virus. However, STING-knockout cancer cells infected with the oncolytic virus showed impaired immunogenicity, as immunogenic cell death was improperly triggered. In agreement with these observations, STING-knockout tumors raised in a murine syngeneic model were more resistant to a combined treatment of the oncolytic virus with PD-1 blockade. The present study demonstrates the antitumor benefit of antiviral immunity and sheds lights on the mechanisms of immune resistance to oncovirotherapy exerted by STING-loss in tumor cells.

Abstract

The dichotomic contribution of cancer cell lysis and tumor immunogenicity is considered essential for effective oncovirotherapy, suggesting that the innate antiviral immune response is a hurdle for efficacy of oncolytic viruses. However, emerging evidence is resizing this view. By sensing cytosolic DNA, the cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING) axis can both counteract viral spread and contribute to the elicitation of adaptive immunity via type I interferon responses. In this paper, we analyzed the tumor-resident function of Sting-mediated DNA sensing in a combined approach of oncovirotherapy and PD-1 immune checkpoint blockade, in an immunocompetent murine model. While supporting increased lytic potential by oncolytic HER2-retargeted HSV-1 in vitro and in vivo, Sting-knockout tumors showed molecular signatures of an immunosuppressive tumor microenvironment. These signatures were correspondingly associated with ineffectiveness of the combination therapy in a model of established tumors. Results suggest that the impairment in antiviral response of Sting-knockout tumors, while favoring viral replication, is not able to elicit an adequate immunotherapeutic effect, due to lack of immunogenic cell death and the inability of Sting-knockout cancer cells to promote anti-tumor adaptive immune responses. Accordingly, we propose that antiviral, tumor-resident Sting provides fundamental contributions to immunotherapeutic efficacy of oncolytic viruses.

The role of extracellular vesicles in COVID-19 virus infection

Extracellular vesicles releasing from various types of cells contribute to intercellular communication via delivering bio-molecules like nucleic acids, proteins, and lipids to recipient cells. Exosomes are 30-120 nm extracellular vesicles that participate in several pathological conditions. Virus-infected cells release exosomes that are implicated in infection through transferring viral components such as viral-derived miRNAs and proteins. As well, exosomes contain receptors for viruses that make recipient cells susceptible to virus entry. Since December 2019, SARS-CoV-2 (COVID-19) infection has become a worldwide urgent public health concern. There is currently no vaccine or specific antiviral treatment existing for COVID-19 virus infection. Hence, it is critical to find a safe and effective therapeutic tool to patients with severe COVID-19 virus infection. Extracellular vesicles may contribute to spread this virus as they transfer such receptors as CD9 and ACE2, which make recipient cells susceptible to virus docking. Upon entry, COVID-19 virus may be directed into the exosomal pathway, and its component is packaged into exosomes for secretion. Exosome-based strategies for the treatment of COVID-19 virus infection may include following items: inhibition of exosome biogenesis and uptake, exosome-therapy, exosome-based drug delivery system, and exosome-based vaccine. Mesenchymal stem cells can suppress nonproductive inflammation and improve/repair lung cells including endothelial and alveolar cells, which damaged by COVID-19 virus infection. Understanding molecular mechanisms behind extracellular vesicles related COVID-19 virus infection may provide us with an avenue to identify its entry, replication, spreading, and infection to overcome its adverse effects.

New Insights on the Mobility of Viral and Host Non-Coding RNAs Reveal Extracellular Vesicles as Intriguing Candidate Antiviral Targets

Intercellular communication occurring by cell-to-cell contacts and via secreted messengers trafficked through extracellular vehicles is critical for regulating biological functions of multicellular organisms. Recent research has revealed that non-coding RNAs can be found in extracellular vesicles consistent with a functional importance of these molecular vehicles in virus propagation and suggesting that these essential membrane-bound bodies can be highjacked by viruses to promote disease pathogenesis. Newly emerging evidence that coronaviruses generate non-coding RNAs and use extracellular vesicles to facilitate viral pathogenicity may have important implications for the development of effective strategies to combat COVID-19, a disease caused by infection with the novel coronavirus, SARS-CoV-2. This article provides a short overview of our current understanding of the interactions between non-coding RNAs and extracellular vesicles and highlights recent research which supports these interactions as potential therapeutic targets in the development of novel antiviral therapies.

Extracellular Vesicles in the Pathogenesis of Viral Infections in Humans

Most cells can release extracellular vesicles (EVs), membrane vesicles containing various proteins, nucleic acids, enzymes, and signaling molecules. The exchange of EVs between cells facilitates intercellular communication, amplification of cellular responses, immune response modulation, and perhaps alterations in viral pathogenicity. EVs serve a dual role in inhibiting or enhancing viral infection and pathogenesis. This review examines the current literature on EVs to explore the complex role of EVs in the enhancement, inhibition, and potential use as a nanotherapeutic against clinically relevant viruses, focusing on neurotropic viruses: Zika virus (ZIKV) and human immunodeficiency virus (HIV). Overall, this review's scope will elaborate on EV-based mechanisms, which impact viral pathogenicity, facilitate viral spread, and modulate antiviral immune responses.

Extracellular vesicles in virus infection and pathogenesis

Viruses are obligate intracellular parasites that usurp cellular signaling networks to promote pathogen spread and disease progression. Signaling through extracellular vesicles (EVs) is an emerging field of study in the virus-host interaction network. EVs relay information both locally and distally through incorporated contents, typically without tripping innate immune sensors. Therefore, this extracellular signaling axis presents itself as a tantalizing target for promoting a favorable niche for the pathogen(s) takeover of the host, particularly for chronic infections. From the incorporation of virus-encoded molecules such as micro RNAs and proteins/enzymes to the envelopment of entire infectious particles, evolutionary distinct viruses have shown a remarkable ability to converge on this means of communication. In this review, we will cover the recent advances in this field and explore how EV can be used as potential biomarkers for chronic, persistent, or latent virus infections.

Generation of Small RNA-Modulated Exosome Mimetics for Bone Regeneration

Administration of exosomes is considered an attractive cell-free approach to skeletal repair and pathological disease treatment. However, poor yield for the production technique and unexpected therapeutic efficacy of exosomes have been obstacles to their widespread use in clinical practices. Here, we report an alternative strategy to produce exosome-related vesicles with high yields and improved regenerative capability. An extrusion approach was employed to amass exosome mimetics (EMs) from human mesenchymal stem cells (hMSCs). The collected EMs had a significantly increased proportion of vesicles positive for the exosome-specific CD-63 marker compared with MSC-derived exosomes. EMs were further obtained from genetically modified hMSCs in which expression of noggin, a natural bone morphogenetic protein antagonist, was down-regulated to enhance osteogenic properties of EMs. Moreover, the administration of hMSC-EMs in conjunction with an injectable chitosan hydrogel into mouse nonhealing calvarial defects demonstrated robust bone regeneration. Importantly, mechanistic studies revealed that the enhanced osteogenesis by EMs in which noggin was suppressed was mediated via inhibition of miR-29a. These findings demonstrate the great promise of MSC-mediated EMs and modulation of small RNA signaling for skeletal regeneration and cell-free therapy.

Different miRNA contents between mammary epithelial cells and milk fat globules: a random or a targeted process?

MicroRNAs (miRNAs) are small noncoding RNAs present in milk-derived extracellular vesicles and milk fat globules (MFG). Nucleic acid content between the lactating mammary tissue (MT) and MFG are quite similar but discrepancies exist in their miRNA content. Our objective was to identify the origin of these discrepancies, and to evaluate the existence of a possible mechanism sorting miRNAs that will or will not be exported from the mammary epithelial cells (MECs) in bovine MFG. miR-125b-5p, miR-126-3p, miR-141-3p, and miR-204-5p, chosen on the basis of their abundance in the MT, were quantified using RT-qPCR in lactating cow MT, MFG, and laser capture-microdissected MECs. Two miRNAs (miR-125b-5p and miR-141-3p) were detected in the MT as well as in MFG and MECs. miR-204-5p was detected only in the MT, suggesting that it is very likely expressed in a cell type other than MECs. miR-126-3p was detected both in the MT and in MECs but not in MFG, suggesting a targeting mechanism for miRNAs in MECs. These results highlights differences in miRNA content between MECs and MFG may be due to a possibly not random mechanism for loading MFG with miRNA cargos that could involve a variable distribution in MECs or a sorting mechanism.

Modern Techniques for the Isolation of Extracellular Vesicles and Viruses

Extracellular signaling is pivotal to maintain organismal homeostasis. A quickly emerging field of interest within extracellular signaling is the study of extracellular vesicles (EV), which act as messaging vehicles for nucleic acids, proteins, metabolites, lipids, etc. from donor cells to recipient cells. This transfer of biologically active material within a vesicular body is similar to the infection of a cell through a virus particle, which transfers genetic material from one cell to another to preserve an infection state, and viruses are known to modulate EV. Although considerable heterogeneity exists within EV and viruses, this review focuses on those that are small (< 200 nm in diameter) and of relatively low density (< 1.3 g/mL). A multitude of isolation methods for EV and virus particles exist. In this review, we present an update on methods for their isolation, purification, and phenotypic characterization. We hope that the information we provide will be of use to basic science and clinical investigators, as well as biotechnologists in this emerging field. Graphical Abstract.

Purification Methods and the Presence of RNA in Virus Particles and Extracellular Vesicles

The fields of extracellular vesicles (EV) and virus infections are marred in a debate on whether a particular mRNA or non-coding RNA (i.e., miRNA) is packaged into a virus particle or copurifying EV and similarly, whether a particular mRNA or non-coding RNA is contained in meaningful numbers within an EV. Key in settling this debate, is whether the purification methods are adequate to separate virus particles, EV and contaminant soluble RNA and RNA:protein complexes. Differential centrifugation/ultracentrifugation and precipitating agents like polyethylene glycol are widely utilized for both EV and virus purifications. EV are known to co-sediment with virions and other particulates, such as defective interfering particles and protein aggregates. Here, we discuss how encased RNAs from a heterogeneous mixture of particles can be distinguished by different purification methods. This is particularly important for subsequent interpretation of whether the RNA associated phenotype is contributed solely by virus or EV particles or a mixture of both. We also discuss the discrepancy of miRNA abundance in EV from different input material.

Exosomal miR-145-5p derived from orthohantavirus-infected endothelial cells inhibits HTNV infection

Human infection of orthohantavirus can cause potentially fatal diseases, such as hemorrhagic fever with renal syndrome (HFRS) caused by Hantaan virus (HTNV) in Eurasia. Exosomes are new carriers for information exchange between cells. Cumulative findings suggest that exosomes released from parental infected cells can block or promote viral infection in recipient cells, but the role of exosomes in hantavirus infection is poorly understood. In our study, we identified the exosomes derived from HTNV-infected human vascular endothelial cells (HUVECs) (Exo-HV) and found the antiviral properties of Exo-HV in the uninfected recipient cells. High-throughput sequencing revealed the distinctly expressed miRNAs transcriptomes in Exo-HV. MiR-145-5p, one of the abundant miRNAs packaged into Exo-HV, was found to be able to transferred to recipient cells and functioned by directly targeting M RNA of HTNV 76-118 and inducing type I interferon (IFN-I) response, thus, blocking the viral replication. Concluding, this study indicated that exosomes released by HTNV-infected HUVECs were able to transfer active molecules, miR-145-5p as a proving sample, to mediate novel anti-HTNV activity in the neighboring uninfected cells, which will help us to explore new strategies for the treatment of infectious disease utilizing exosomes with miRNA.

Human Cytomegalovirus Utilizes Extracellular Vesicles To Enhance Virus Spread

Human cytomegalovirus (HCMV) manipulates cellular processes associated with secretory pathways within an infected cell to facilitate efficient viral replication. However, little is known about how HCMV infection alters the surrounding cellular environment to promote virus spread to uninfected cells. Extracellular vesicles (EVs) are key signaling molecules that are commonly altered in numerous disease states. Previous reports have shown that viruses commonly alter EVs, which can significantly impact infection. This study finds that HCMV modulates EV biogenesis machinery through upregulation of the endosomal sorting complex required for transport (ESCRT) proteins. This regulation appears to increase the activity of EV biogenesis, since HCMV-infected fibroblasts have increased vesicle release and altered vesicle size compared to EVs from uninfected cells. EVs generated through ESCRT-independent pathways are also beneficial to virus spread in fibroblasts, as treatment with the EV inhibitor GW4869 slowed the efficiency of HCMV spread. Importantly, the transfer of EVs purified from HCMV-infected cells enhanced virus spread. This suggests that HCMV modulates the EV pathway to transfer proviral signals to uninfected cells that prime the cellular environment for incoming infection and enhance the efficiency of virus spread.IMPORTANCE Human cytomegalovirus (HCMV) is a herpesvirus that leads to serious health consequences in neonatal or immunocompromised patients. Clinical management of infection in these at-risk groups remains a serious concern even with approved antiviral therapies available. It is necessary to increase our understanding of the cellular changes that occur during infection and their importance to virus spread. This may help to identify new targets during infection that will lead to the development of novel treatment strategies. Extracellular vesicles (EVs) represent an important method of intercellular communication in the human host. This study finds that HCMV manipulates this pathway to increase the efficiency of virus spread to uninfected cells. This finding defines a new layer of host manipulation induced by HCMV infection that leads to enhanced virus spread.

Extracellular Vesicles in Viral Infections of the Nervous System

Almost all types of cells release extracellular vesicles (EVs) into the extracellular space. EVs such as exosomes and microvesicles are membrane-bound vesicles ranging in size from 30 to 1000 nm in diameter. Under normal conditions, EVs mediate cell to cell as well as inter-organ communication via the shuttling of their cargoes which include RNA, DNA and proteins. Under pathological conditions, however, the number, size and content of EVs are found to be altered and have been shown to play crucial roles in disease progression. Emerging studies have demonstrated that EVs are involved in many aspects of viral infection-mediated neurodegenerative diseases. In the current review, we will describe the interactions between EV biogenesis and the release of virus particles while also reviewing the role of EVs in various viral infections, such as HIV-1, HTLV, Zika, CMV, EBV, Hepatitis B and C, JCV, and HSV-1. We will also discuss the potential uses of EVs and their cargoes as biomarkers and therapeutic vehicles for viral infections.

Use of exosomes as vectors to carry advanced therapies

Exosomes are microvesicles of nanometric size involved in the communication between cells and tissues. Inside their bilipidic membrane they carry nucleic acids such as cargos (DNA, miRNA, etc.). Some of the advantages that make exosomes very attractive therapeutic vehicles are (i) their tropism through different tissues, (ii) the ability to pass biological barriers and (iii) the protection of the encapsulated material from the immune system and degradation. Viruses are some of the most widely employed gene therapy vehicles; however, they are still facing many problems, such as inefficient tropism to damaged areas and their elimination by the immune system. One of the functions attributed to exosomes is the elimination of substances that could be harmful to the cell, including viruses. Recently it has been investigated whether complete viruses or part of them could be encapsulated in exosomes, for a new viral-exosome gene therapy approach. Moreover, nanotechnology is another type of advanced therapy (together with gene and cell therapies) that can be used, among other utilities, to transfer genetic material. Recently the field of encapsulation of nanomaterials in exosomes, with or without gene transfer, is increasing. In this review we will summarize all of those studies.

hnRNPA2B1 Associated with Recruitment of RNA into Exosomes Plays a Key Role in Herpes Simplex Virus 1 Release from Infected Cells

hnRNPA2B1, an abundant cellular protein, has been reported to recruit RNAs bearing a specific sequence (EXO motif) into exosomes. We characterized an exosome population averaging 100 ± 50 nm in diameter and containing a defined set of constitutive exosome markers. This population packages microRNAs (miRNAs) and can be directed to block targeted gene expression in a dose-dependent fashion. The objective of this study was to characterize the role of hnRNPA2B1 in the recruitment of miRNA. We report the following four key findings. (i) hnRNPA2B1 is not a component of exosomes produced in HEp-2 or HEK293T cells. Hence, hnRNPA2B1 carries its cargo, at most, to the site of exosome assembly, but it is not itself incorporated into exosomes. (ii) The accumulation of exosomes produced by cells in which the gene encoding hnRNPA2B1 has been knocked out (ΔhnRNPA2B1 cells) was reduced 3-fold. (iii) In uninfected HEp-2 cells, hnRNPA2B1 is localized in the nucleus. In cells infected with herpes simplex virus 1 (HSV-1), hnRNPA2B1 was quantitatively exported to the cytoplasm and at least a fraction of hnRNPA2B1 colocalized with a Golgi marker. (iv) Lastly, in ΔhnRNPA2B1 cells, there was a 2- to 3-fold reduction in virus yield but a significant (>10-fold) reduction in HSV-1 released through the apical surface into the extracellular environment. The absence of hnRNPA2B1 had no significant impact on the basolateral export of HSV-1 from infected to uninfected cells by direct cell-to-cell contact. The results suggest that hnRNPA2B1 plays a key role in the transport of enveloped virus from its site of assembly to the extracellular environment.IMPORTANCE In this report, we show that hnRNPA2B1 is not a component of exosomes produced in HEp-2 or HEK293T cells. In herpes simplex virus 1 (HSV-1)-infected cells, hnRNPA2B1 was quantitatively translocated from the nucleus into the cytoplasm. In infected ΔhnRNPA2B1 cells, Golgi-dependent transport of virus from the apical surface to the extracellular medium was significantly reduced. In essence, this report supports the hypothesis that hnRNPA2B1 plays a key role in the egress of exosomes and HSV-1 from infected cells.

Extracellular Vesicles in Viral Spread and Antiviral Response

Viral spread by both enveloped and non-enveloped viruses may be mediated by extracellular vesicles (EVs), including microvesicles (MVs) and exosomes. These secreted vesicles have been demonstrated to be an efficient mechanism that viruses can use to enter host cells, enhance spread or evade the host immune response. However, the complex interplay between viruses and EVs gives rise to antagonistic biological tasks-to benefit the viruses, enhancing infection and interfering with the immune system or to benefit the host, by mediating anti-viral responses. Exosomes from cells infected with herpes simplex type 1 (HSV-1) may transport viral and host transcripts, proteins and innate immune components. This virus may also use MVs to expand its tropism and evade the host immune response. This review aims to describe the current knowledge about EVs and their participation in viral infection, with a specific focus on the role of exosomes and MVs in herpesvirus infections, particularly that of HSV-1.

MicroRNA Biomarkers for Infectious Diseases: From Basic Research to Biosensing

In the pursuit of improved diagnostic tests for infectious diseases, several classes of molecules have been scrutinized as prospective biomarkers. Small (18–22 nucleotide), non-coding RNA transcripts called microRNAs (miRNAs) have emerged as promising candidates with extensive diagnostic potential, due to their role in numerous diseases, previously established methods for quantitation and their stability within biofluids. Despite efforts to identify, characterize and apply miRNA signatures as diagnostic markers in a range of non-infectious diseases, their application in infectious disease has advanced relatively slowly. Here, we outline the benefits that miRNA biomarkers offer to the diagnosis, management, and treatment of infectious diseases. Investigation of these novel biomarkers could advance the use of personalized medicine in infectious disease treatment, which raises important considerations for validating their use as diagnostic or prognostic markers. Finally, we discuss new and emerging miRNA detection platforms, with a focus on rapid, point-of-care testing, to evaluate the benefits and obstacles of miRNA biomarkers for infectious disease.

Cloaked Viruses and Viral Factors in Cutting Edge Exosome-Based Therapies

Extracellular vesicles (EVs) constitute a heterogeneous group of vesicles released by all types of cells that play a major role in intercellular communication. The field of EVs started gaining attention since it was realized that these vesicles are not waste bags, but they carry specific cargo and they communicate specific messages to recipient cells. EVs can deliver different types of RNAs, proteins, and lipids from donor to recipient cells and they can influence recipient cell functions, despite their limited capacity for cargo. EVs have been compared to viruses because of their size, cell entry pathways, and biogenesis and to viral vectors because they can be loaded with desired cargo, modified, and re-targeted. These properties along with the fact that EVs are stable in body fluids, they can be produced and purified in large quantities, they can cross the blood-brain barrier, and autologous EVs do not appear to cause major adverse effects, have rendered them attractive for therapeutic use. Here, we discuss the potential for therapeutic use of EVs derived from virus infected cells or EVs carrying viral factors. We have focused on six major concepts: (i) the role of EVs in virus-based oncolytic therapy or virus-based gene delivery approaches; (ii) the potential use of EVs for developing viral vaccines or optimizing already existing vaccines; (iii) the role of EVs in delivering RNAs and proteins in the context of viral infections and modulating the microenvironment of infection; (iv) how to take advantage of viral features to design effective means of EV targeting, uptake, and cargo packaging; (v) the potential of EVs in antiviral drug delivery; and (vi) identification of novel antiviral targets based on EV biogenesis factors hijacked by viruses for assembly and egress. It has been less than a decade since more attention was given to EV research and some interesting concepts have already been developed. In the coming years, additional information on EV biogenesis, how they are hijacked and utilized by pathogens, and their impact on the microenvironment of infection is expected to indicate avenues to optimize existing therapeutic tools and develop novel approaches.

The Role of Extracellular Vesicles as Allies of HIV, HCV and SARS Viruses

Extracellular vesicles (EVs) are lipid bilayer-enclosed entities containing proteins and nucleic acids that mediate intercellular communication, in both physiological and pathological conditions. EVs resemble enveloped viruses in both structural and functional aspects. In full analogy with viral biogenesis, some of these vesicles are generated inside cells and, once released into the extracellular milieu, are called “exosomes”. Others bud from the plasma membrane and are generally referred to as “microvesicles”. In this review, we will discuss the state of the art of the current studies on the relationship between EVs and viruses and their involvement in three important viral infections caused by HIV, HCV and Severe Acute Respiratory Syndrome (SARS) viruses. HIV and HCV are two well-known pathogens that hijack EVs content and release to create a suitable environment for viral infection. SARS viruses are a new entry in the world of EVs studies, but are equally important in this historical framework. A thorough knowledge of the involvement of the EVs in viral infections could be helpful for the development of new therapeutic strategies to counteract different pathogens.

Research Advances in How the cGAS-STING Pathway Controls the Cellular Inflammatory Response

Double-stranded DNA (dsDNA) sensor cyclic-GMP-AMP synthase (cGAS) along with the downstream stimulator of interferon genes (STING) acting as essential immune-surveillance mediators have become hot topics of research. The intrinsic function of the cGAS-STING pathway facilitates type-I interferon (IFN) inflammatory signaling responses and other cellular processes such as autophagy, cell survival, senescence. cGAS-STING pathway interplays with other innate immune pathways, by which it participates in regulating infection, inflammatory disease, and cancer. The therapeutic approaches targeting this pathway show promise for future translation into clinical applications. Here, we present a review of the important previous works and recent advances regarding the cGAS-STING pathway, and provide a comprehensive understanding of the modulatory pattern of the cGAS-STING pathway under multifarious pathologic states.

Role of RNA-binding proteins during the late stages of Flavivirus replication cycle

The genus Flavivirus encompasses several worldwide-distributed arthropod-borne viruses including, dengue virus, Japanese encephalitis virus, West Nile virus, yellow fever virus, Zika virus, and tick-borne encephalitis virus. Infection with these viruses manifest with symptoms ranging from febrile illness to life- threatening hypotensive shock and encephalitis. Therefore, flaviviruses pose a great risk to public health. Currently, preventive measures are falling short to control epidemics and there are no antivirals against any Flavivirus.Flaviviruses carry a single stranded positive-sense RNA genome that plays multiple roles in infected cells: it is translated into viral proteins, used as template for genome replication, it is the precursor of the subgenomic flaviviral RNA and it is assembled into new virions. Furthermore, viral RNA genomes are also packaged into extracellular vesicles, e.g. exosomes, which represent an alternate mode of virus dissemination.Because RNA molecules are at the center of Flavivirus replication cycle, viral and host RNA-binding proteins (RBPs) are critical determinants of infection. Numerous studies have revealed the function of RBPs during Flavivirus infection, particularly at the level of RNA translation and replication. These proteins, however, are also critical participants at the late stages of the replication cycle. Here we revise the function of host RBPs and the viral proteins capsid, NS2A and NS3, during the packaging of viral RNA and the assembly of new virus particles. Furthermore, we go through the evidence pointing towards the importance of host RBPs in mediating cellular RNA export with the idea that the biogenesis of exosomes harboring Flavivirus RNA would follow an analogous pathway.

Herpes simplex virus 1 regulates β-catenin expression in TG neurons during the latency-reactivation cycle

When herpes simplex virus 1 (HSV-1) infection is initiated in the ocular, nasal, or oral cavity, sensory neurons within trigeminal ganglia (TG) become infected. Following a burst of viral transcription in TG neurons, lytic cycle viral genes are suppressed and latency is established. The latency-associated transcript (LAT) is the only viral gene abundantly expressed during latency, and LAT expression is important for the latency-reactivation cycle. Reactivation from latency is required for virus transmission and recurrent disease, including encephalitis. The Wnt/β-catenin signaling pathway is differentially expressed in TG during the bovine herpesvirus 1 latency-reactivation cycle. Hence, we hypothesized HSV-1 regulates the Wnt/β-catenin pathway and promotes maintenance of latency because this pathway enhances neuronal survival and axonal repair. New studies revealed β-catenin was expressed in significantly more TG neurons during latency compared to TG from uninfected mice or mice latently infected with a LAT-/- mutant virus. When TG explants were incubated with media containing dexamethasone to stimulate reactivation, significantly fewer β-catenin+ TG neurons were detected. Conversely, TG explants from uninfected mice or mice latently infected with a LAT-/- mutant increased the number of β-catenin+ TG neurons in the presence of DEX relative to samples not treated with DEX. Impairing Wnt signaling with small molecule antagonists reduced virus shedding during explant-induced reactivation. These studies suggested β-catenin was differentially expressed during the latency-reactivation cycle, in part due to LAT expression.

Variability in protein cargo detection in technical and biological replicates of exosome-enriched extracellular vesicles

Exosomes are extracellular vesicles (EVs) of ~20-200 nm diameter that shuttle DNAs, RNAs, proteins and other biomolecules between cells. The large number of biomolecules present in exosomes demands the frequent use of high-throughput analysis. This, in turn, requires technical replicates (TRs), and biological replicates (BRs) to produce accurate results. As the number and abundance of identified biomolecules varies between replicates (Rs), establishing the replicate variability predicted for the event under study is essential in determining the number of Rs required. Although there have been few reports of replicate variability in high throughput biological data, none of them focused on exosomes. Herein, we determined the replicate variability in protein profiles found in exosomes released from 3 lung adenocarcinoma cell lines, H1993, A549 and H1975. Since exosome isolates are invariably contaminated by a small percentage of ~200-300 nm microvesicles, we refer to our samples as exosome-enriched EVs (EE-EVs). We generated BRs of EE-EVs from each cell line, and divided each group into 3 TRs. All Rs were analyzed by liquid chromatography/mass spectrometry (LC/MS/MS) and customized bioinformatics and biostatistical workflows (raw data available via ProteomeXchange: PXD012798). We found that the variability among TRs as well as BRs, was largely qualitative (protein present or absent) and higher among BRs. By contrast, the quantitative (protein abundance) variability was low, save for the H1975 cell line where the quantitative variability was significant. Importantly, our replicate strategy identified 90% of the most abundant proteins, thereby establishing the utility of our approach.

Herpes Simplex Virus 1 Spread in Oligodendrocytic Cells Is Highly Dependent on MAL Proteolipid

Herpes simplex virus 1 (HSV-1) is a neurotropic pathogen that can infect many types of cells and establish latent infections in neurons. HSV-1 may spread from infected to uninfected cells by two main routes: by cell-free virus or by cell-to-cell spread. In the first case, virions exit into the extracellular space and then infect another cell from the outside. In the second case, viral transmission occurs through cell-to-cell contacts via a mechanism that is still poorly understood. A third mode of spread, using extracellular vesicles, also exists. In this study, we demonstrate the important role for a myelin protein, myelin and lymphocyte protein (MAL), in the process of cell-to-cell viral spread in oligodendrocytes. We show that MAL is involved in trafficking of virions along cell processes and that MAL depletion produces a significant alteration in the viral cycle, which reduces cell-to cell spread of HSV-1.

Myelin and lymphocyte protein (MAL) is a tetraspan integral membrane protein that resides in detergent-insoluble membrane fractions enriched in condensed membranes. MAL is expressed in oligodendrocytes, in Schwann cells, where it is essential for the stability of myelin, and at the apical membrane of epithelial cells, where it has a critical role in transport. In T lymphocytes, MAL is found at the immunological synapse and plays a crucial role in exosome secretion. However, no involvement of MAL in viral infections has been reported so far. Here, we show that herpes simplex virus 1 (HSV-1) virions travel in association with MAL-positive structures to reach the end of cellular processes, which contact uninfected oligodendrocytes. Importantly, the depletion of MAL led to a significant decrease in infection, with a drastic reduction in the number of lytic plaques in MAL-silenced cells. These results suggest a significant role for MAL in viral spread at cell contacts. The participation of MAL in the cell-to-cell spread of HSV-1 may shed light on the involvement of proteolipids in this process.

IMPORTANCE Herpes simplex virus 1 (HSV-1) is a neurotropic pathogen that can infect many types of cells and establish latent infections in neurons. HSV-1 may spread from infected to uninfected cells by two main routes: by cell-free virus or by cell-to-cell spread. In the first case, virions exit into the extracellular space and then infect another cell from the outside. In the second case, viral transmission occurs through cell-to-cell contacts via a mechanism that is still poorly understood. A third mode of spread, using extracellular vesicles, also exists. In this study, we demonstrate the important role for a myelin protein, myelin and lymphocyte protein (MAL), in the process of cell-to-cell viral spread in oligodendrocytes. We show that MAL is involved in trafficking of virions along cell processes and that MAL depletion produces a significant alteration in the viral cycle, which reduces cell-to cell spread of HSV-1.

Isolation/Analysis of Extracellular Microvesicles from HSV-1-Infected Cells

Extracellular vesicles (EVs) are secreted membrane vesicles, derived from endosomes or from the plasma membrane, which have been isolated from most cell types and biological fluids. Although EVs are highly heterogeneous and their classification is complex, two major categories can be distinguished: microvesicles (MVs), which derive from the shedding of the plasma membrane, and exosomes, which correspond to intraluminal vesicles released to the extracellular milieu upon fusion of multivesicular bodies (MVBs) with the plasma membrane. Cells infected with viruses may secrete MVs containing viral proteins, RNAs and, in some instances, infectious virions. A recent study carried out by our laboratory has shown that MVs released by cells infected with HSV-1 contained virions and were endocytosed by naïve cells leading to a productive infection. This suggests that HSV-1 may use MVs for spreading, expanding its tropism and evading the host immune response. Here we describe in detail the methods used to isolate and analyse the MVs released from HSV-1-infected cells.

Herpes Simplex Virus 1 MicroRNA miR-H28 Exported to Uninfected Cells in Exosomes Restricts Cell-to-Cell Virus Spread by Inducing Gamma Interferon mRNA

An earlier report showed that herpes simplex virus 1 (HSV-1) expresses two microRNAs (miRNAs), miR-H28 and miR-H29, late in the infectious cycle. The miRNAs are packed in exosomes and, in recipient cells, restrict the transmission of virus from infected cells to uninfected cells. We now report that (i) miR-H28 induced the synthesis of gamma interferon (IFN-γ) in both infected cells and cells transfected with miR-H28, (ii) IFN-γ accumulated concurrently with viral proteins in infected cells, (iii) IFN-γ was produced in HEp-2 cells derived from cancer tissue and in HEK293T cells derived from normal tissue, and (iv) HSV-1 replication was affected by exposure to IFN-γ before infection but not during or after infection. The results presented in this report support the growing body of evidence indicating that HSV-1 encodes functions designed to reduce the spread of infection from infected cells to uninfected cells, possibly in order to maximize the transmission of virus from infected individuals to uninfected individuals.IMPORTANCE In this report, we show that IFN-γ is produced by HSV-1 viral miR-H28 and viral replication is blocked in cells exposed to IFN-γ before infection but not during or after infection. The inevitable conclusion is that HSV-1 induces IFN-γ to curtail its spread from infected cells to uninfected cells. In essence, this report supports the hypothesis that HSV-1 encodes functions that restrict the transmission of virus from cell to cell.

Bioinformatically-predicted varicella zoster virus small non-coding RNAs are expressed in lytically-infected epithelial cells and neurons

Most herpesviruses use both host and viral small non-coding RNAs (sncRNA), especially microRNA, to modulate infection. Bioinformatic analyses of NGS data obtained from Varicella Zoster virus (VZV)-infected cells predicted 24 VZVsncRNA, seven of which were confirmed to be expressed in infected fibroblasts and neurons using stem-loop quantitative reverse transcription PCR (SL-PCR). We here assayed for the expression of all 24 of the bioinformatically predicted VZVsncRNA in cells productively infected by VZV using SL-PCR. 23 of the 24 predicted sequences were detected in VZV-infected ARPE19 cells and 19 of the 24 sequences in infected human neurons generated by two methods from embryonic stem cells. We also show that blocking one of two newly-tested VZV-encoded sncRNA using locked nucleotide antagonists significantly increased viral replication. These findings suggest that further study of VZV encoded sncRNA could elucidate an additional level of regulation of the life cycle of this pathogenic human herpesvirus.

The Metabolic Profile of Tumor and Virally Infected Cells Shapes Their Microenvironment Counteracting T Cell Immunity

Upon activation naïve T cells undergo metabolic changes to support the differentiation into subsets of effector or regulatory cells, and enable subsequent metabolic adaptations to form memory. Interfering with these metabolic alterations leads to abrogation or reprogramming of T cell differentiation, demonstrating the importance of these pathways in T cell development. It has long been appreciated that the conversion of a healthy cell to a cancerous cell is accompanied by metabolic changes, which support uncontrolled proliferation. Especially in solid tumors these metabolic changes significantly influence the tumor microenvironment (TME) and affect tumor infiltrating immune cells. The TME is often hypoxic and nutrient depleted, additionally tumor cells produce co-inhibitory signals, together suppressing the immune response. Interestingly, viruses can stimulate a metabolism akin to that seen in tumor cells in their host cells and even in neighboring cells (e.g., via transfer of virally modified extracellular vesicles). Thus, viruses create their own niche which favors viral persistence and propagation, while again keeping the immune response at bay. In this review we will focus on the mechanisms employed by tumor cells and viruses influencing T cell metabolic regulation and the impact they have on shaping T cell fate.

Modern Techniques for the Isolation of Extracellular Vesicles and Viruses

Extracellular signaling is pivotal to maintain organismal homeostasis. A quickly emerging field of interest within extracellular signaling is the study of extracellular vesicles (EV), which act as messaging vehicles for nucleic acids, proteins, metabolites, lipids, etc. from donor cells to recipient cells. This transfer of biologically active material within a vesicular body is similar to the infection of a cell through a virus particle, which transfers genetic material from one cell to another to preserve an infection state, and viruses are known to modulate EV. Although considerable heterogeneity exists within EV and viruses, this review focuses on those that are small (< 200 nm in diameter) and of relatively low density (< 1.3 g/mL). A multitude of isolation methods for EV and virus particles exist. In this review, we present an update on methods for their isolation, purification, and phenotypic characterization. We hope that the information we provide will be of use to basic science and clinical investigators, as well as biotechnologists in this emerging field. Graphical Abstract.

The Role of Extracellular Vesicles in Viral Infection and Transmission

Extracellular vesicles (EVs) have been found to be released by any type of cell and can be retrieved in every circulating body fluid, namely blood (plasma, serum), saliva, milk, and urine. EVs were initially considered a cellular garbage disposal tool, but later it became evident that they are involved in intercellular signaling. There is evidence that viruses can use EV endocytic routes to enter uninfected cells and hijack the EV secretory pathway to exit infected cells, thus illustrating that EVs and viruses share common cell entry and biogenesis mechanisms. Moreover, EVs play a role in immune response against viral pathogens. EVs incorporate and spread both viral and host factors, thereby prompting or inhibiting immune responses towards them via a multiplicity of mechanisms. The involvement of EVs in immune responses, and their potential use as agents modulating viral infection, will be examined. Although further studies are needed, the engineering of EVs could package viral elements or host factors selected for their immunostimulatory properties, to be used as vaccines or tolerogenic tools in autoimmune diseases.

Intercellular communication in the innate immune system through the cGAS-STING pathway

Type I interferons (IFNα/β) are pivotal to anti-viral defense. Although an absolute requirement for viral clearance, the presence of IFN in high doses sustained over time has been implicated to be the underlying cause of clinical outcomes such as interferonopathies. Therefore, tight regulation of cellular communication processes is required for tissue homeostasis. This review covers the cellular signaling pathways initiated by DNA sensing through cGAS in the cellular cytosol, with particular focus on the subsequent establishment of immune responses in bystander cells.

Differential Requirements for gE, gI, and UL16 among Herpes Simplex Virus 1 Syncytial Variants Suggest Unique Modes of Dysregulating the Mechanism of Cell-to-Cell Spread

Like all the herpesviruses, herpes simplex virus encodes machinery that enables it to move through cell junctions to avoid neutralizing antibodies. This cell-to-cell spread mechanism requires the viral fusion machinery (gD, gH/gL, and gB) and numerous accessory proteins. Of all of these, minor alterations to only four proteins (gB, gK, UL20, or UL24) will dysregulate the fusion machinery, allowing the formation of syncytia. In contrast, removal of individual accessory proteins will block cell-to-cell spread, forcing the virus to transmit in a cell-free manner. In the context of a Syn variant, removal of a required accessory protein will block cell fusion, again forcing cell-free spread. This has been investigated most thoroughly for gBsyn variants, which lose their syncytial phenotype in the absence of several accessory proteins, including gE, gI, UL16, and UL21, which are known to physically interact. Recently it was found that UL21 is not needed for gKsyn-, UL20syn-, or UL24syn-induced cell fusion, and hence it was of interest to ascertain whether gE, gI, and UL16 are required for Syn variants other than gBsyn. Null mutants of these were each combined with seven syncytial variants distributed among gK, UL20, and UL24. Surprisingly, very different patterns of accessory protein requirements were revealed. Indeed, for the three gKsyn variants tested, two different patterns were found. Also, three mutants were able to replicate without causing cytopathic effects. These findings show that mutations that produce Syn variants dysregulate the cell-to-cell-spread machinery in unique ways and provide clues for elucidating how this virus moves between cells.IMPORTANCE Approximately 2/3 of adults worldwide are latently infected with herpes simplex virus 1. Upon reactivation, the virus has the ability to evade neutralizing antibodies by moving through cell junctions, but the mechanism of direct cell-to-cell spread is poorly understood. The machinery that assembles between cells includes the viral fusion proteins and various accessory proteins that prevent cells from fusing. Alterations in four proteins will dysregulate the machinery, allowing neighboring cells to fuse to make syncytia, but this can be prevented by removing various individual accessory proteins to further disable the machinery. Previously, the accessory protein UL21 was found to be important for the activity of some syncytial variants but not others. In this study, we discovered that UL16, gE, and gI all act differently in how they control the fusion machinery. A better understanding of the mechanism of cell-to-cell spread may enable the development of drugs that block it.

Delivery of microRNAs by Extracellular Vesicles in Viral Infections: Could the News be Packaged?

Extracellular vesicles (EVs) are released by various cells and recently have attracted attention because they constitute a refined system of cell-cell communication. EVs deliver a diverse array of biomolecules including messenger RNAs (mRNAs), microRNAs (miRNAs), proteins and lipids, and they can be used as potential biomarkers in normal and pathological conditions. The cargo of EVs is a snapshot of the donor cell profile; thus, in viral infections, EVs produced by infected cells could be a central player in disease pathogenesis. In this context, miRNAs incorporated into EVs can affect the immune recognition of viruses and promote or restrict their replication in target cells. In this review, we provide an updated overview of the roles played by EV-delivered miRNAs in viral infections and discuss the potential consequences for the host response. The full understanding of the functions of EVs and miRNAs can turn into useful biomarkers for infection detection and monitoring and/or uncover potential therapeutic targets.

Innate Sensing of DNA Virus Genomes

DNA viruses are linked to many infectious diseases and contribute significantly to human morbidity and mortality worldwide. Moreover, DNA viral infections are usually lifelong and hard to eradicate. Under certain circumstances, these viruses can cause fatal disease, especially in children and immunocompromised patients. An efficient innate immune response against these viruses is critical, not only as the first line of host defense against viral infection but also for mounting more specific and robust adaptive immunity against the virus. Recognition of the viral DNA genome is the very first step of this whole process and is crucial for understanding viral pathogenesis as well as for preventing and treating DNA virus-associated diseases. This review focuses on the current state of our knowledge on how human DNA viruses are sensed by the host innate immune system and how viral proteins counteract this immune response.

Exosomal Interferon-Induced Transmembrane Protein 2 Transmitted to Dendritic Cells Inhibits Interferon Alpha Pathway Activation and Blocks Anti-Hepatitis B Virus Efficacy of Exogenous Interferon Alpha

The negative regulators in the interferon (IFN) signaling pathway inhibit intrahepatic immune response, resulting in suboptimal therapeutic response to IFNα treatment in chronic hepatitis B (CHB) patients. Identifying the key negative factors and elucidating the regulating mechanism are essential for improving anti-HBV (hepatitis B virus) efficacy of IFNα. From the Gene Expression Omnibus (GEO) database, we downloaded and analyzed gene expression profiles of CHB patients with different responses to IFNα (GSE54747), and found that innate immune status was associated with the IFNα-based therapeutic response in CHB patients. Through PCR array, we found higher baseline level of IFN-induced transmembrane protein 2 (IFITM2) mRNA and lower baseline level of IFNα mRNA in peripheral blood mononuclear cells (PBMCs) of CHB patients with suboptimal response to IFNα treatment. Increased IFITM2 protein was also found in the serum of IFNα nonresponsive patients. With further experiments, we found that overexpressing IFITM2 in Huh7 cells suppressed endogenous IFNα synthesis by inhibiting phosphorylation of extracellular signal-regulated kinase (ERK), TANK-binding kinase 1 (TBK1), and interferon regulatory factor 3 (IRF3); knocking out IFITM2 enhanced activation of the endogenous IFNα synthesis pathway, exhibiting better inhibition on HBV replication. We also found that IFITM2 protein was shuttled by exosomes to dendritic cells (DCs), the main source of endogenous IFNα. Exosome-mediated transport of IFITM2 inhibited synthesis of endogenous IFNα in DCs whereas the inhibitory effect was abolished when IFITM2 was knocked out. Furthermore, we demonstrated that both palmitoylation inhibitor and mutation on 70/71 sites of IFITM2 protein influenced its incorporation into exosomes. Mutated IFITM2 protein increased the effect of IFNα against HBV. Conclusion: Exosome-mediated transport of IFITM2 to DCs inhibits IFNα pathway activation and blocks anti-HBV efficacy of exogenous IFNα. The findings provide an explanation to the suboptimal response of CHB patients to IFNα treatment.

Advances in therapeutic applications of extracellular vesicles

Extracellular vesicles (EVs) are nanometer-sized, lipid membrane-enclosed vesicles secreted by most, if not all, cells and contain lipids, proteins, and various nucleic acid species of the source cell. EVs act as important mediators of intercellular communication that influence both physiological and pathological conditions. Given their ability to transfer bioactive components and surmount biological barriers, EVs are increasingly being explored as potential therapeutic agents. EVs can potentiate tissue regeneration, participate in immune modulation, and function as potential alternatives to stem cell therapy, and bioengineered EVs can act as delivery vehicles for therapeutic agents. Here, we cover recent approaches and advances of EV-based therapies.

Extracellular Vesicles: Delivery Vehicles of Myokines

Movement and regular physical activity are two important factors that help the human body prevent, reduce and treat different chronic diseases such as obesity, type 2 diabetes, heart diseases, hypertension, sarcopenia, cachexia and cancer. During exercise, several tissues release molecules into the blood stream, and are able to mediate beneficial effects throughout the whole body. In particular, contracting skeletal muscle cells have the capacity to communicate with other organs through the release of humoral factors that play an important role in the mechanisms of adaptation to physical exercise. These muscle-derived factors, today recognized as myokines, act as endocrine and paracrine hormones. Moreover, exercise may stimulate the release of small membranous vesicles into circulation, whose composition is influenced by the same exercise. Combining the two hypotheses, these molecules related to exercise, named exer-kines, might be secreted from muscle cells inside small vesicles (nanovesicles). These could act as messengers in tissue cross talk during physical exercise. Thanks to their ability to deliver useful molecules (such as proteins and miRNA) in both physiological and pathological conditions, extracellular vesicles can be thought of as promising candidates for potential therapeutic and diagnostic applications for several diseases.

Exosome-Mediated Delivery of Inducible miR-423-5p Enhances Resistance of MRC-5 Cells to Rabies Virus Infection

The human diploid cell line Medical Research Council -5 (MRC-5) is commonly utilized for vaccine development. Although a rabies vaccine developed in cultured MRC-5 cells exists, the poor susceptibility of MRC-5 cells to the rabies virus (RABV) infection limits the potential yield of this vaccine. The underlying mechanism of MRC-5 cell resistance to RABV infection remains unknown. In this study, we demonstrate that viral infection increased exosomal release from MRC-5 cells; conversely, blocking exosome release promoted RABV infection in MRC-5 cells. Additionally, RABV infection up-regulated microRNA (miR)-423-5p expression in exosomes, resulting in feedback inhibition of RABV replication by abrogating the inhibitory effect of suppressor of cytokine signaling 3 (SOCS3) on type I interferon (IFN) signaling. Furthermore, intercellular delivery of miR-423-5p by exosomes inhibited RABV replication in MRC-5 cells. We also show that RABV infection increased IFN-β production in MRC-5 cells and that blocking the type I IFN receptor promoted RABV infection. In conclusion, MRC-5 cells were protected from RABV infection by the intercellular delivery of exosomal miR-423-5p and the up-regulation of IFN-β. These findings reveal novel antiviral mechanisms in MRC-5 cells against RABV infection. miR-423-5p, exosomes, and IFN signaling pathways may therefore be potential targets for improving MRC-5 cell-based rabies vaccine production.

GADD45γ Activated Early in the Course of Herpes Simplex Virus 1 Infection Suppresses the Activation of a Network of Innate Immunity Genes

The stress response genes encoding GADD45γ, and to a lesser extent GADD45β, are activated early in infection with herpes simplex virus 1 (HSV-1). Cells that had been depleted of GADD45γ by transfection of short hairpin RNA (shRNA) or in which the gene had been knocked out (ΔGADD45γ) yielded significantly less virus than untreated infected cells. Consistent with lower virus yields, the ΔGADD45γ cells (either uninfected or infected with HSV-1) exhibited significantly higher levels of transcripts of a cluster of innate immunity genes, including those encoding IFI16, IFIT1, MDA5, and RIG-I. Members of this cluster of genes were reported by this laboratory to be activated concurrently with significantly reduced virus yields in cells depleted of LGP2 or HDAC4. We conclude that innate immunity to HSV-1 is normally repressed in unstressed cells and repression appears to be determined by two mechanisms. The first, illustrated here, is through activation by HSV-1 infection of the gene encoding GADD45γ. The second mechanism requires constitutively active expression of LGP2 and HDAC4.IMPORTANCE Previous studies from our laboratory reported that knockout of some innate immunity genes was associated with increases in the expression of overlapping networks of genes and significant loss of the ability to support the replication of HSV-1; knockout of other genes was associated with decreases in the expression of overlapping networks of genes and had no effect on virus replication. In this report, we document that depletion of GADD45γ reduced virus yields concurrently with significant upregulation of the expression of a cluster of innate immunity genes comprising IFI16, IFIT1, MDA5, and RIG-I. This report differs from the preceding study in an important respect; i.e., the preceding study found no evidence to support the hypothesis that HSV-1 maintained adequate levels of LGP2 or HDAC4 to block upregulation of the cluster of innate immunity genes. We show that HSV-1 causes upregulation of the GADD45γ gene to prevent the upregulation of innate immunity genes.

Extracellular vesicles from Kaposi Sarcoma-associated herpesvirus lymphoma induce long-term endothelial cell reprogramming

Extracellular signaling is a mechanism that higher eukaryotes have evolved to facilitate organismal homeostasis. Recent years have seen an emerging interest in the role of secreted microvesicles, termed extracellular vesicles (EV) or exosomes in this signaling network. EV contents can be modified by the cell in response to stimuli, allowing them to relay information to neighboring cells, influencing their physiology. Here we show that the tumor virus Kaposi's Sarcoma-associated herpesvirus (KSHV) hijacks this signaling pathway to induce cell proliferation, migration, and transcriptome reprogramming in cells not infected with the virus. KSHV-EV activates the canonical MEK/ERK pathway, while not alerting innate immune regulators, allowing the virus to exert these changes without cellular pathogen recognition. Collectively, we propose that KSHV establishes a niche favorable for viral spread and cell transformation through cell-derived vesicles, all while avoiding detection.

Biogenesis of Extracellular Vesicles during Herpes Simplex Virus 1 Infection: Role of the CD63 Tetraspanin

Herpes simplex virus 1 (HSV-1) infections afflict more than 80% of the population worldwide. The virus primarily infects mucoepithelial cells and establishes latent reservoirs in neurons in sensory ganglia. Frequent reactivation has been linked to severe diseases, especially in immunocompromised individuals. Earlier, we reported that viral and host factors are packaged in extracellular vesicles (EVs) and delivered to uninfected cells, where they activate antiviral responses and restrict virus infection. Here, we interrogated the effect of HSV-1 infection on EV biogenesis. We found that HSV-1 infection causes a decrease in the amount of intracellular CD63 protein with a concomitant increase in extracellular CD63. This observation correlates with our previous finding that infected cells release more CD63-positive EVs than uninfected cells. The stimulation of CD63 exocytosis requires virus replication. CD63 is a member of the tetraspanin family of proteins that traffics between the plasma membrane and endosomal compartments and has a role in sorting cargo into the EVs. Previously, we reported that in cells depleted of CD63, HSV-1 virus yields increased, and here we provide data showing that in cells overexpressing CD63, HSV-1 virus yields decreased. Taken together, our data indicate that CD63 negatively impacts HSV-1 infection and that the CD63-positive EVs could control the dissemination of the virus in the host. Perhaps EV release by HSV-1-infected cells is a mechanism that controls virus dissemination.IMPORTANCE Intercellular communication, especially in neurons, largely relies on EVs, and modulation of EVs is known to impact physiological processes. Here, we present evidence that HSV-1 infection causes major alterations in the biogenesis of EVs, including an increase in their number and an increase in the CD63-positive population of EVs. These alterations result in an enrichment of the milieu of infection with EVs carrying signatures from infected cells. In addition to changes in the origin and type, EVs released by infected cells have differences in cargo, as they carry viral and host factors determined by the virus. The tetraspanin CD63 negatively impacts the infection, as demonstrated by CD63-knockdown and overexpression assays. A proposed mechanism involves the activation of antiviral responses in cells receiving CD63-positive EVs released by infected cells. Overall, HSV-1 causes major alterations in EVs that could contribute to HSV-1 persistence and pathogenesis.

Extracellular Vesicles in Herpes Viral Spread and Immune Evasion

Extracellular vesicles (EVs) are involved in numerous processes during infections by both enveloped and non-enveloped viruses. Among them, herpes simplex virus type-1 (HSV-1) modulates secretory pathways, allowing EVs to exit infected cells. Many characteristics regarding the mechanisms of viral spread are still unidentified, and as such, secreted vesicles are promising candidates due to their role in intercellular communications during viral infection. Another relevant role for EVs is to protect virions from the action of neutralizing antibodies, thus increasing their stability within the host during hematogenous spread. Recent studies have suggested the participation of EVs in HSV-1 spread, wherein virion-containing microvesicles (MVs) released by infected cells were endocytosed by naïve cells, leading to a productive infection. This suggests that HSV-1 might use MVs to expand its tropism and evade the host immune response. In this review, we briefly describe the current knowledge about the involvement of EVs in viral infections in general, with a specific focus on recent research into their role in HSV-1 spread. Implications of the autophagic pathway in the biogenesis and secretion of EVs will also be discussed.

Varicella-Zoster Virus and the Enteric Nervous System

Varicella zoster virus (VZV) infects and becomes latent in sensory, enteric, and other autonomic neurons during the viremia of varicella. Reactivation of VZV in neurons that project to the skin causes the rash of zoster; however, reactivation of VZV in enteric neurons can cause a painful gastrointestinal disorder ("enteric zoster") without cutaneous manifestations. Detection of VZV DNA in saliva of patients with gastrointestinal symptoms may suggest enteric zoster. This diagnosis is reinforced by observing a response to antiviral therapy and can be confirmed by detecting VZV gene products in intestinal mucosal biopsies. We developed an in vivo guinea pig model that may be useful in studies of VZV latency and reactivation. VZV-infected lymphocytes are used to induce latent infection in sensory and enteric neurons; evidence suggests that exosomes and stimulator of interferon genes (STING) may, by preventing proliferation play roles in the establishment of neuronal latency.

Herpes simplex virus 1 miRNA sequence variations in latently infected human trigeminal ganglia

Human herpes simplex virus 1 (HSV-1) expresses numerous miRNAs, the function of which is not well understood. Several qualitative and quantitative analyses of HSV-1 miRNAs have been performed on infected cells in culture and animal models, however, there is very limited knowledge of their expression in human samples. We sequenced small-RNA libraries of RNA derived from human trigeminal ganglia latently infected with HSV-1 and Varicella zoster virus (VZV) and detected only a small subset of HSV-1 miRNA. The most abundantly expressed miRNAs are miR-H2, miRNA that regulates the expression of immediate early gene ICP0, and miR-H3 and -H4, both miRNAs expressed antisense to the transcript encoding the major neurovirulence factor ICP34.5. The sequence of many HSV-1 miRNAs detected in human samples was different from the sequences deposited in miRBase, which might significantly affect targeted functional analyses.

Extracellular Vesicles Released by Herpes Simplex Virus 1-Infected Cells Block Virus Replication in Recipient Cells in a STING-Dependent Manner

Herpes simplex virus 1 (HSV-1)-infected cells release extracellular vesicles (EVs) that deliver to uninfected cells viral factors and host components, such as the stimulator of interferon genes (STING), which activates type I interferon upon foreign DNA sensing. The functions of EVs released by HSV-1-infected cells have remained unknown. Here, we describe a procedure to separate the EVs from HSV-1 virions that is based on an iodixanol/sucrose gradient. STING, along with the EV markers CD63 and CD9, was found in light-density fractions, while HSV components accumulated in heavy-density fractions. HSV-1 infection stimulated the release of EVs from the cells. The EVs derived from infected cells, but not from uninfected cells, activated innate immunity in recipient cells and suppressed viral gene expression and virus replication. Moreover, only the EVs derived from infected cells stimulated the expression of a subset of M1-type markers in recipient macrophages. Conversely, EVs derived from STING-knockdown cells failed to stimulate the expression of these M1-type markers, they activated innate immune responses to a lesser extent in recipient cells, and they did not sustain the inhibition of virus replication. These data suggest that STING from the EV donor cells contributes to the antiviral responses in cells receiving EVs from HSV-1-infected cells. Perturbations in the biogenesis of EVs by silencing CD63 or blocking the activity of the neutral spingomyelinase-2 (nSMase-2) increased the HSV-1 yields. Overall, our data suggest that the EVs released from HSV-1-infected cells negatively impact the infection and could control the dissemination of the virus.IMPORTANCE Extracellular vesicles (EVs) are released by all types of cells as they constitute major mechanism of intercellular communication and have the capacity to alter the functions of recipient cells despite their limited capacity for cargo. How the EVs released by HSV-infected cells could alter the surrounding microenvironment and influence the infection currently remains unknown. The cargo of EVs reflects the physiological state of the cells in which they were produced, so the content of EVs originating from infected cells is expected to be substantially different from that of healthy cells. Our studies indicate that the EVs released by HSV-1-infected cells carry innate immune components such as STING and other host and viral factors; they can activate innate immune responses in recipient cells and inhibit HSV-1 replication. The implication of these data is that the EVs released by HSV-1-infected cells could control HSV-1 dissemination promoting its persistence in the host.

Viral antigens detectable in CSF exosomes from patients with retrovirus associated neurologic disease: functional role of exosomes

BACKGROUND

HTLV-1 infects over 20 million people worldwide and causes a progressive neuroinflammatory disorder in a subset of infected individuals called HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP). The detection of HTLV-1 specific T cells in the cerebrospinal fluid (CSF) suggests this disease is immunopathologically mediated and that it may be driven by viral antigens. Exosomes are microvesicles originating from the endosomal compartment that are shed into the extracellular space by various cell types. It is now understood that several viruses take advantage of this mode of intercellular communication for packaging of viral components as well. We sought to understand if this is the case in HTLV-1 infection, and specifically if HTLV-1 proteins can be found in the CSF of HAM/TSP patients where we know free virus is absent, and furthermore, if exosomes containing HTLV-1 Tax have functional consequences.

RESULTS

Exosomes that were positive for HTLV-1 Tax by Western blot were isolated from HAM/TSP patient PBMCs (25/36) in ex vivo cultures by trapping exosomes from culture supernatants. HTLV-1 seronegative PBMCs did not have exosomes with Tax (0/12), (Fisher exact test, p = 0.0001). We were able to observe HAM/TSP patient CSF (12/20) containing Tax⁺ exosomes but not in HTLV-1 seronegative MS donors (0/5), despite the absence of viral detection in the CSF supernatant (Fisher exact test p = 0.0391). Furthermore, exosomes cultivated from HAM/TSP PBMCs were capable of sensitizing target cells for HTLV-1 specific CTL lysis.

CONCLUSION

Cumulatively, these results show that there are HTLV-1 proteins present in exosomes found in virus-free CSF. HAM/TSP PBMCs, particularly CD4⁺CD25⁺ T cells, can excrete these exosomes containing HTLV-1 Tax and may be a source of the exosomes found in patient CSF. Importantly, these exosomes are capable of sensitizing an HTLV-1 specific immune response, suggesting that they may play a role in the immunopathology observed in HAM/TSP. Given the infiltration of HTLV-1 Tax-specific CTLs into the CNS of HAM/TSP patients, it is likely that exosomes may also contribute to the continuous activation and inflammation observed in HAM/TSP, and may suggest future targeted therapies in this disorder.